Get Ad-Aware -
http://www.lavasoft.de with the latest reference file (they have an automatic updater tool there, too) and run it. Could be spyware/malware/adware causing this issue.
If no joy, in IE go to Tools...Internet Options...Advanced tab, Browsing section, uncheck "Enable third-party browser extensions", click Apply, click Okay, reboot. If that solves your problem, then more troubleshooting is needed to find out exactly which program, or Browser Helper Object (BHO) is causing this problem. You don't want to leave it at that, as some BHOs are useful or necessary - like Adobe Acrobat for reading .pdf files or an essential component of Norton AV. Get BHO Cop:
http://www.pcmag.com/article2/0,4149,2023,00.asp - read all about BHOs, download link is on the right, under Contents. Then uncheck all items, and then gradually replace one or two at a time to narrow down the culprit.
If no joy, go to Start...Run, "msconfig", StartUp tab and see if there is anything there that looks suspicious. You will also see the path where that program is starting from.
See this site for more information on pop-ups and possible hijacking of your system:
http://www.mvps.org/inetexplorer/Darnit.htm#pop_up